A mirror is an object that reflects light in a way that preserves much of its original quality subsequent to its contact with the mirror. Two type of mirrors are second surface mirrors (or back surface mirrors) and first surface mirrors (or front surface mirrors). Second surface mirrors typically include a glass substrate with a reflective coating on a back surface thereof (i.e., not on the front surface which is first hit by incoming light). Incoming light passes through the glass substrate of a second surface mirror before being reflected by the coating. Thus, reflected light passes through the glass substrate of a second surface mirror twice; once before being reflected and again after being reflected by the mirror coating on its way to a viewer or other element(s). In certain instances, passing through the glass substrate twice can create ambiguity in directional reflection and imperfect reflections (such as double reflected images) may sometimes result. Second surface mirrors also tend to have reduced reflection compared to first surface mirrors, because the light passes twice through the glass substrate. Mirrors such as bathroom and bedroom mirrors are typically second surface mirrors so that the glass substrate can be used to protect the reflective coating provided on the back surface.
In first surface mirrors, a reflective coating is provided on the front surface of a substrate (i.e., the surface which is first hit by incoming light) so that incoming light is reflected by the coating before it passes through the substrate. Since the light to be reflected does not pass through the substrate (e.g., glass substrate) in first surface mirrors, a first surface mirror generally has a higher reflectance and fewer imperfect reflections than a second surface mirror.
Light waves emit an electric field perpendicular to the wave's direction of travel. An electric field which rotates as a light wave travels is known as circular or elliptical polarization (depending on the pattern of rotation). A linearly polarized light wave emits an electric field which is oriented in a single direction.
When light is reflected, the plane made by the propagation direction and a vector perpendicular to the plane of a reflecting surface is referred to as the plane of incidence. One of the coordinate systems used to describe the polarization of light refers to the component of the electric field parallel to the plane of incidence as p-polarized light and the component perpendicular to the plane of incidence as s-polarized light.
FIG. 1 illustrates a cross sectional view of a related art first surface mirror, including a glass substrate, an aluminum (Al) layer, a MgF2 layer, and a PrTiOx layer. The related art first surface mirror of FIG. 1 has at least the following disadvantages: Exposure to MgF2 is known to be detrimental to human health. Like many other rare earth elements, praseodymium is of low to moderate toxicity. Both PrTiOx and MgF2 cannot be easily sputtered in large-area coaters. MgF2 is costly. MgF2 is a soft material which tends to lack mechanical and environmental durability.
Laser scanners and copiers (sometimes combined as one unit) are known in the art. However, it is desirable to provide good quality mirrors in such laser scanner and/or copiers. Laser scanners have been known to utilize one or more first surface mirrors.
U.S. Pat. No. 6,934,085 discloses in the background section a first surface mirror having a structure: glass/Al/SiO2/TiO2. Unfortunately, a first surface mirror structured as glass/Al(40 nm)/SiO2(80 nm)/TiO2(40 nm) does not provide adequate reflectance of both s-polarized and p-polarized light for laser scanner and/or copier applications. For example, such a first surface mirror with these layer thicknesses does not have a difference between s-polarization reflectance and p-polarization reflectance, at a wavelength of about 780 nm, of less than or equal to about 3% percent at an angles of incidence of from 0 to 60°.